In the past, a displacement detector has been proposed to detect a change in impedance of a coil portion caused by inserting a core into the coil portion, thereby providing a signal indicative of position data of the core relative to the coil portion.
This kind of displacement detector is disclosed in U.S. Pat. Nos. 5,003,258, 4,864,232, and 5,893,300. For example, as shown in FIG. 43, in a position detector disclosed in U.S. Pat. No. 5,003,258, a coil portion 2 is formed such that a change in impedance Z (inductance component) with respect to temperature resulting from a magnetic material 21 of a core 1 is cancelled by a change in impedance Z (eddy current component) with respect to temperature resulting from a nonmagnetic material 22. That is, against a problem that a temperature coefficient of the impedance Z of the coil portion depends on displacement of the core 1, the dependency of the temperature coefficient of the impedance Z on the displacement is reduced by modifying structures of the coil portion and its circumference. However, it leads to an increase in the total number of parts, and makes difficult to carry out the positioning of the parts. In addition, there are problems of a reduction in degree of freedom of design of the coil portion, increase in cost performance, and deterioration in versatility.
In addition, as another prior art, Japanese Patent Early Publication [kokai] 2000-186903 discloses a non-contact type position sensor using high-frequency magnetic field. As shown in FIG. 44, this position sensor is provided with a rotating shaft 201 rotated in conjunction with an object to be detected, arc-shaped, movable metal body 202 coupled with the rotating shaft 201 through a coupling member 202c to be moved on the circumference of a circle having the axis of the rotating shaft 201 as its center in conjunction with the rotation of the rotating shaft, a pair of sensor coils 203 (203a, 203b) fixed on the circumference of the circle, and having a center axis curved such that arms (202a, 202b) of the movable metal body 202 can come in and go out of through holes of the respective sensor coils to change an occupied area in a magnetic path, a sensor circuit for detecting a magnetic change caused depending on the occupied area of the arms 202a, 202b in the sensor coils 203a, 203b by activating those sensor coils to generate a RF modulated magnetic field, and resistances 205a, 205b for detecting coil currents flowing in the sensor coils 203a, 203b. According to this sensor, since the arms 202a, 202b of the movable metal body 202 are complementarily inserted in the sensor coils 203a, 203b, and the sensor circuit 204 detects a change in impedance of each of the sensor coils 203a, 203b, there is an advantage that accurate positioning is not needed.
However, in an angle status where the arms 202a, 202b of the movable metal body 202 are not inserted in the sensor coils 203a, 203b, the coil impedances of the sensor coils 203a, 203b are only the impedance of the coil wire. Therefore, temperature characteristics (temperature coefficients) thereof depend on only a factor of the coil wire. On the other hand, in an angle status where the arms 202a, 202b are inserted in the sensor coils 203a, 203b, the coil impedance of the sensor coils 203a, 203b has an increase in coil impedance (an increase in inductance, eddy current loss, hysteresis loss, and so on) caused by the insertions of the arms 202a, 202b, in addition to the impedance of the coil wire. In this case, since the temperature coefficient is determined by a sum of different temperature coefficients of the factors causing the increase in impedance, it is different from the temperature coefficient in the angle status where the arms 202a, 202b are not inserted in the sensor coils 203a, 203b. This means that the temperature coefficient changes depending on the insertion amount (angle). Even if digital trimming is performed to achieve a temperature compensation by a compensation circuit 210 of the sensor circuit 204, there is a problem that an accurate output can not be provided to ECU unless a complicated compensation for changing an amplification rate in response to the angle is performed. Thus, the conventional sensor still has room for improvement.